Many circuits in electronic equipment provide protection from the harmful effects of overvoltages, overcurrents, etc. These protection circuits are often designed as an integral part of the general electronic circuit, but may be added thereto as ancillary devices or circuits.
Protection circuits may often be constructed on silicon substrates, such as bipolar transistors, diodes or thyristors. Silicon bipolar devices can carry large magnitudes of current and thus are well adapted for use in protecting electronic circuits from damage by overvoltages and overcurrents. Solid state bipolar devices constructed with junctions have an inherent capacitance that is a function of the width of the depletion region. The depletion region in a semiconductor junction functions as the “dielectric” layer of a capacitor. Since the width of the depletion region varies with the voltage impressed across the junction, the capacitance of a bipolar semiconductor junction varies as a function of the voltage applied across the junction. Capacitors whose values vary with voltage are inherently nonlinear devices. In other words, a bipolar overvoltage protection device placed across a circuit to be protected can affect the operation of the circuit even if the overvoltage protection device remains in its off state. The non-linearity can lead to suboptimal channel performance and intermodulation distortion.
The adverse affects of the foregoing are experienced in many applications, including communication lines where overvoltage protection circuits are routinely employed to protect transmitting and receiving circuits from high voltages that may be inadvertently coupled to the communication lines. Many devices in the thyristor family can be employed to respond to the overvoltage condition and provide a low impedance path between the communication line and ground, or other path where the energy is safely dissipated.
The adverse affects of the use of silicon bipolar overvoltage protection devices may not arise from the fact that such devices have an inherent capacitance, but rather from the characteristic that the capacitance changes as a function of the voltage, frequency and temperature to which the device is subjected. As an example, many communication lines are adapted for carrying high speed digital signals of various protocols, including ADSL, T1, E1, ADSL2+, ADSL2++, 10BaseT, VDSL, VDSL2, T3, 100BaseT and others. Many of these protocols are carried between remote destinations by way of modems or other transmission and receiving circuits. In order to optimize the transmission of high speed data, many modems utilize an initial process of selecting the proper equalization components so that the digital signals can be transmitted at the highest speed permitted by the frequency response of the line and the circuits associated with the line. The equalization parameters selected by the modem are those that exist at the time equalization testing is carried out. This is usually once when the modem is placed in service, and on each reboot thereof after initial operation. It can be seen that if the electrical state of the line changes after the equalization session, the transmission data rate may not be optimized, and thus transmission errors can occur.
An example of transmission inefficiencies can arise in connection with the following example. A modem placed on line or booted into operation will be programmed to automatically carry out an equalization process for determining the best electrical parameters to be switched into operation to optimize high speed data transmission. The modem will be connected to the communication line, such as a telephone DSL line adapted for carrying VDSL or other data signals. An on-hook state (of the telephone set) of the DSL line for carrying digital signals is typically 48 volts. After the modem has completed the equalization process, it is situated to provide optimum transmission of the VDSL signals, based on the electrical characteristics of the DSL communication line that existed during the equalization process. Typically, the modem will adapt the voltage magnitude of the digital signals as a function of the length of the communication line so that the lowest power level is achieved while yet minimizing the transmission data error rate.
During an actual communication session by a user in which the VDSL signals are being transmitted at a high rate, assume that the user's telephone set connected to the same DSL communication line is placed in an off-hook condition. In other words, the user is simultaneously using the DSL communication line for both verbal communications with the telephone set, and for data communications using the modem. This off-hook condition places a different set of voltages on the communication line. The communication line goes from a 48-volt on-hook state to about a 10-volt off-hook state. As such, the capacitance of the overvoltage protection devices, and possibly other devices, will change with changing voltages, thus modifying the electrical characteristics of the lines to which the modem was equalized. With the communication line now having different electrical characteristics, the effective transmission rate may be lowered, but the modem keeps transmitting at the rate optimized during the equalization session. As a result, the data receiver or modem at the receiving end of the communication line may detect errors arising from the transmission of data at a rate higher than the line can reliably carry in the off-hook condition. The excessive error rate may cause the modem to retrain, which results in a temporary loss of service during the retraining session. This is generally unacceptable and annoying to the user.
From the foregoing, it can be seen that a need exists for a technique for making overvoltage protection devices and circuits less prone to changes in capacitance as a function of voltage, and thereby reduce the change in electrical characteristics of the devices or circuits connected to the lines. Another need exists for an overvoltage protection circuit that includes a bias voltage applied to the overvoltage protection device to minimize changes in capacitance as a function of voltage applied across the device, and in which the magnitude of the bias voltage need not be greater than the line voltages experienced by the line to be protected. Still another need exists for an overvoltage protection circuit in which a bias voltage is applied to an overvoltage protection device for minimizing the change in capacitance of the device as a function of both voltage and frequency applied across the device, and also as a function of the temperature of the overvoltage protection device.